1. Field of the Invention
The present invention relates to a compound semiconductor apparatus and a method for manufacturing the apparatus in which a compound semiconductor layer, a gate insulating layer and a gate electrode are arranged in that order.
2. Description of the Prior Art
A metal-semiconductor field effect transistor (MESFET) and a high electron mobility transistor (HEMT), in which a gallium-arsenic (GaAs) compound semiconductor is used, has been recently watched as a very high speed device. The GaAs-MESFET is currently used for a mobile communication device such as a portable telephone, and the HEMT is currently used for a satellite broadcasting receiving antenna.
Also, a GaAs metal-insulator semiconductor (MIS) FET has been recently watched as a low consumed electric power and high output type compound semiconductor device.
FIG. 1 is a cross sectional view showing a configuration of a conventional GaAs MISFET.
As shown in FIG. 1, an undoped GaAs buffer layer 32 is arranged on a GaAs semiconductor substrate 31, and a GaAs active layer 33 in which an n-type impurity is doped is arranged on the GaAs buffer layer 32. Also, a source electrode 34a and a drain electrode 34b separated from each other are arranged on the GaAs active layer 33, a gate insulating layer 35 placed between the source electrode 34a and the drain electrode 34b is arranged on the GaAs active layer 33, and a gate electrode 36 is arranged on the gate insulating layer 35.
In a prior art, SiO.sub.2 or Al.sub.2 O.sub.3 is used as a material of the gate insulating layer 35. However, when the gate insulating layer 35 is made of SiO.sub.2 or Al.sub.2 O.sub.3, a large number of interface state for electrons and holes are generated in an interface between the gate insulating layer 35 and the GaAs active layer 33. Therefore, there is a drawback that a desired electric characteristic is not obtained in the conventional GaAs MISFET.
To solve this drawback, another prior art in which tertiary-butyl-gallium-sulfide-cubane "((t-Bu)GaS).sub.4 " is used as a source of a gate insulating layer and a GaS insulating layer having a cubic structure, in which a composition ratio of Ga to S is 1:1, is formed by a metal organic chemical vapor deposition (MOCVD) is proposed in a plurality of literatures "A. N. MacInnes et. al., Chem. Mater., 4, 11(1992) ", "A. N. MacInnes et. al., Appl. Phys. Lett., 62, 711(1993)", "M. Tabib-Azar et. al., Appl. Phys. Lett., 63, 625(1993)" and "P. P. Jenkins et. al., Science, 263, 1751(1994)".
In this method disclosed in the above literatures, when a GaS insulating layer is formed on a GaAs layer, a substrate is heated to a temperature ranging from about 400 to 450.degree. C., and GaS is epitaxially grown on the GaAs layer. Because a lattice constant of GaS crystal is close to a lattice constant of GaAs crystal, the number of interface state generated in an interface between the GaAs layer and the GaS layer is considerably decreased. Because the number of interface state is decreased, a non-radiative surface recombination is suppressed, so that a photo luminescence (PL) intensity of GaAs layer with contacting GaS layer is larger than a PL intensity of GaAs layer without contacting GaS layer.
However, though a lattice constant of a normal GaS crystal is 5.4 .ANG., a lattice constant of the GaS crystal epitaxially grown on the GaAs layer is increased to 5.63 .ANG.. That is, the GaS layer and GaAs layer is distorted when the GaS layer is epitaxially grown on the GaAs layer, so that the reduction of the number of interface state is not sufficient in the method disclosed in the above literatures.
Also, in cases where InGaAs is used as a source of the compound semiconductor layer, when a GaS insulating layer is formed on an InGaAs layer by the method disclosed in the above literatures, because a lattice constant of InGaAs crystal is larger than that of GaAs crystal, there is a probability that an interface between the compound semiconductor layer (or the InGaAs layer) and the GaS insulating layer is moreover distorted or a dislocation occurs in the InGaAs layer. Therefore, in cases where the compound semiconductor layer is made of a source other than GaAs, it is not adequate to apply the method disclosed in the above literatures to the formation of the GaS insulating layer.
In addition, in cases where the GaS insulating layer is formed by the MOCVD method, when a substrate temperature reaches 450.degree. C. or more, sulfur (S) atoms existing in the GaS insulating layer are liberated from a GaS structure of the GaS insulating layer, and a composition ratio of Ga to S in the GaS insulating layer is changed. Therefore, it is difficult that the GaS insulating layer has a stable electric characteristic.